is another source.
KNO3 is also sold as stump remover. Read the label, though. Other materials are also sold as stump remover. I do not know if this will be cheaper than buying it on line and paying for shipping, though. (especially if you are already buying other things and paying shipping anyway)
Magnesium is available as Epsom salt, and this will be cheapest in Dollar Tree and similar stores. You might not need magnesium, though, if the GH of your water is already where you want it. This does not supply calcium.
Calcium is available as swimming pool chemicals. (Calcium carbonate) If you have friends with a pool or hot tub, and you need calcium, go read some labels and see if they will give you a few tablespoons. (Calcium carbonate will also raise the KH)
Is your tap water an acceptable source of GH?
Whatever source you find to add Ca and Mg (if you need to at all) make sure you are adding a balanced ratio. Plants seem to use Ca and Mg in a ratio of about 3 or 4 parts Ca to 1 part Mg. You might find GH booster locally, too, and not have to pay shipping. Again, read the labels. Some GH booster also contains potassium, so you would not have to buy K2SO4, if you need enough GH booster.
I would look at the sale forum here at Planted Tank, too. I think there is at least one, probably more, people selling these materials. Maybe one will have some sort of 'package deal' with perhaps 1/4 lb of each or something. Price it out carefully, though. It might really end up cheaper to buy it in larger amounts (such as a pound at a time).
With a high tech tank I would get these materials in a ratio something like:
1 lb KNO3
1/4 lb KH2PO4
1 lb K2SO4 (unless you already know you will be using enough of the first two to supply enough K)
1/4 lb traces
1/4 lb chelated iron
GH booster only if you know you need it. (And adjust the K2SO4 purchase as needed)
If you are not going to do the large water changes of the EI method, then you might start with perhaps 50% of the lower dosing suggested, and test to be sure that is OK. Be prepared to do some water changes if even 50% is too high. With fertilizer already in the substrate the rooted plants may not take as much from the water, but the fertilizer in the water will get into the substrate, and will replace whatever the plants are using. Then it ends up in the plants anyway.
Near me are at least 2 places where people can take household chemicals (paint, fertilizer, cleaning supplies and so on) and drop them off as an alternative to throwing them away in the dump. These recycling places then make these materials available free to anyone who will use them. I have found pool supplies, aquarium and pond supplies and many other useful things at one of these recyclers.